1. Field of the Invention
Exemplary aspects of the present invention generally relate to a drive transmission unit and an image forming apparatus including the drive transmission unit, and more particularly to an image forming apparatus such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile capabilities, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image bearing member (which may, for example, be a photoconductive drum); an optical writer projects a light beam onto the charged surface of the image bearing member to form an electrostatic latent image on the image bearing member according to the image data; a developing device supplies toner to the electrostatic latent image formed on the image bearing member to render the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image bearing member onto a recording medium or is indirectly transferred from the image bearing member onto a recording medium via an intermediate transfer member; a cleaning device then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the unfixed toner image to fix the unfixed toner image on the recording medium, thus forming the image on the recording medium.
The image bearing member is rotated by a drive source such as a motor. Generally, the speed of the drive source is greater than the target speed of the image bearing member. Thus, a planetary gear decelerator is used to reduce the speed of rotation of the drive source so that the reduced speed is transmitted to the image bearing member.
Typically, known planetary gear decelerators consist of a sun gear, an outer gear, a plurality of planetary gears, and a carrier. The sun gear receives torque from the drive source and rotates. The outer gear is coaxially provided to the same shaft as the sun gear and fixed immovably to the planetary gear decelerator. The plurality of planetary gears is disposed equally spaced around the inner circumference thereof and engages the sun gear and the outer gear. The carrier is rotatable on the same shaft as the sun gear and the outer gear, and rotatively supports the planetary gears. Accordingly, the carrier outputs the torque at a reduced speed.
Helical gears are used for the outer gear, the sun gear, and the planetary gears to increase contact ratio, reduce fluctuation of the speed of a meshing cycle, and reduce noise.
A helical gear is a gear with teeth which are set at an angle relative to the axis of rotation. Thus, as the sun gear rotates, a thrust load is generated in the direction of the rotation axis (thrust direction) between the sun gear and the planetary gears, and between the outer gear and the planetary gears in the planetary gear decelerator. The planetary gears do not shift in the direction of the thrust because the direction of thrust load due to engagement of the planetary gears and the sun gear is opposite to the direction of thrust load due to engagement of the planetary gears and the outer gear, thereby canceling out the thrust loads.
As for the outer gear, even when the outer gear moves in the direction of thrust due to engagement with the planetary gears and contacts an adjacent device in the direction of thrust, the outer gear does not get abraded because the outer gear itself does not rotate. Accordingly, loss of power and abrasion due to heat do not occur.
However, if the sun gear shifts in the direction of thrust due to engagement with the planetary gears and hence comes into contact with a partner member opposite the sun gear in the direction of thrust, the sun gear contacts the partner member while rotating. As a result, loss of power and abrasion occur due to rotational friction at the contact portion where the sun gear and the partner member contact.
In view of the above, Japanese Patent No. 4590299 proposes a planetary gear decelerator in which the sun gear is supported by a fixing member via a shaft bearing to prevent the sun gear from shifting in the direction of thrust. In this configuration, the sun gear is prevented from contacting the partner member. However, power must be maintained even when the thrust load is applied to the shaft bearing via the sun gear, and such a shaft bearing is generally expensive.
In another approach, according to JP-2006-307909-A, a portion of the sun gear that contacts the partner member is made spherical. In this configuration, the area of contact with the partner member is reduced, and thus loss of power and abrasion due to rotational friction can be prevented.
Although advantageous and generally effective for its intended purpose, the portion of the sun gear having the spherical surface gets abraded after extended use, causing loss of power and abrasion. Moreover, according to the results of experiments performed by the present inventors, even when grease was supplied to the spherical portion of the sun gear, the grease spattered due to the centrifugal force of rotation of the sun gear, and hence abrasion of the spherical portion of the sun gear was not prevented.